Start solving 2018 problems

I have also updated get.py to download the problems as
`d<day>.txt` instead of `i<day>.txt`. That allows me
to get the day input via `__input__.replace('.py', '.txt')`
which is a little more concise. I don't know why
I didn't do this earlier.

get.py

@@ -12,12 +12,14 @@ year = sys.argv[1]
 day = sys.argv[2]
 
 session_cookie = get_password('aoc-session-cookie', 'felixm')
+assert session_cookie is not None
+
 url = f"https://adventofcode.com/{year}/day/{day}/input"
 cookies = {'session': session_cookie}
 response = requests.get(url, cookies=cookies)
 
 if response.status_code == 200:
-    filename = f"i{day}.txt"
+    filename = f"d{day}.txt"
     with open(filename, 'w') as file:
         file.write(response.text)
     print(f"Year {year} {filename} written.")